read yaml file use env data or default value

I use yaml encountered some problems

• I want use env data
• I want use default value

I checked a lot of information, and can solve two problems

• in yaml file you can use ${DATABASE_HOST} can incoming.

 DATABASE_HOST=localhost run main.go

• default value in main.go

  var defaultBaseDBConfig = DBConfig {
    Host: "loclhost", 
    Port: "5432", 
    Username: "postgres", 
    Password: "postgres",
    Database: "postgres",
  }

• in this demo, i use two layers yaml

• This is a demo, if you have questions, you can contact me.
• If you use my code, i hope you can give me a start